Inside an engine, oil for lubrication, which is stored in an oil pan, is drawn and feed by an oil pump, and an oil passage for supplying lubricant to each of the lubricated parts is formed. In addition, an oil relief passage is connected to the oil passage downstream of the oil pump, and the oil relief passage is provided with an oil relief valve that opens when the pressure (hydraulic pressure) inside the oil passage that accompanies the feeding of the oil by the oil pump becomes equal to or greater than a set pressure. The excess oil returns to the oil pan, and the maximum hydraulic pressure in the oil passage is thereby regulated.
In connection with this, an oil pump has been proposed in which, during a low temperature startup, when the viscosity of the oil becomes high, the load that is applied to the oil pump is reduced by increasing the oil relief from the oil passage, and thereby an improvement in the startup characteristics is realized (Patent Document 1).
Specifically, when the engine is started up, a flywheel is rotated by a starter motor, and the flywheel must reach a minimum rotating speed that enables the engine to be started-up. However, because the viscosity of the oil becomes high when the temperature is low, the load that is applied to the oil pump increases because the flow resistance of the oil that is flowing through the oil passage increases, and this acts as a reactive force against the starter motor drive power. Thus, the flywheel cannot be made to reach the minimum speed that enables the engine to be started up, and this may cause the startup characteristics to deteriorate.
Thus, according to the oil passage structure that has been proposed in Patent Document 1, a first oil relief passage that communicates the oil passage that is downstream of the oil pump and the oil pan, and a second oil relief passage that is parallel thereto are provided. An oil relief valve that opens when the hydraulic pressure inside the oil passage becomes equal to or greater than a set value is provided on the first oil relief passage, and an oil relief passage opening and closing valve that opens due to temperature and a startup signal is provided on the second oil relief passage.
Due to having this type of structure, the oil relief passage opening and closing valve opens during low temperatures when the viscosity of the oil becomes high, and a portion of the oil is returned to the oil pan via the second oil relief passage. Thus, the load that is applied to the oil pump is reduced because the hydraulic pressure downstream of the oil pump becomes low.
Furthermore, the proposal of Patent Document 2 is one that improves the proposal according to Patent Document 1. Patent Document 2 discloses an oil passage structure for an engine in which, even if there is a change in the oil viscosity that accompanies a change in temperature, the load on the oil pump is reduced while ensuring the minimum hydraulic pressure that is necessary for a normal startup, and the startup characteristics are improved. Specifically, an oil passage structure for an engine that is provided with a first oil relief passage that communicates the oil passage downstream of the oil pump and the oil pan and a second oil relief passage that is parallel thereto, and an oil relief valve that opens when the hydraulic pressure in the oil passage becomes equal to or greater than a set hydraulic pressure is provided on the first oil relief passage and an oil relief passage opening and closing valve that opens due to a startup signal is provided on the second oil relief passage, wherein the second oil relief valve serial to the oil relief passage opening and closing valve is provided on the second oil relief passage, and the valve opening pressure of this second oil relief valve is set so as to be lower than the valve opening pressure of the oil relief valve that is provided on the first oil relief passage and this second oil relief valve opens due to a hydraulic pressure that is equal to or greater than a minimum hydraulic pressure that is necessary during a startup.
Due to this type of structure, the oil relief passage opening and closing valve that is provided on the second oil relief passage is opened, and during a low temperature startup, the load that is applied to the oil pump can be reduced by lowering the hydraulic pressure downstream of the oil pump. In addition, because the opening of the second relief valve is carried out by a hydraulic pressure that is equal to or greater than a minimum hydraulic pressure that is necessary during startup, even when there is a change in the viscosity of the oil, the oil does not return to the oil pan excessively, and the amount of oil that is necessary during a startup is ensured.
However, piston jets are one among the various mechanisms that are incorporated into an engine that appropriately cool the parts of the engine after a warm-up has been completed. These inject oil toward the piston that is in operation to realize cooling of the area around the piston. These piston jets operate such that when the hydraulic pressure inside the oil passage becomes equal to or greater than a predetermined value, nozzles that face the pistons open and oil is injected.
Patent Document 1: Japanese Unexamined Utility Model Application Publication No. 55-135112
Patent Document 2: Japanese Examined Utility Model Application Publication No. 2-34404